Are you sure it was rust ? Very stale gas apparantly can produce a deposit that looks a lot like rust - In May I had to flush the tank on my Fiero and replace fuel pump, filter, and sock because the pump became clogged with something that looked like rust but I do not think it was (when cleaned the metal lines shone and were not pitted at all).

This was a car that had been in my garage with about 3/4 of a tank of gas for two years. Gas that came out smelled bad.